Various methods have been heretofore suggested for the removal of NO.sub.x from NO.sub.x -containing exhaust gases such as those issuing from combustion sources. Among them, the method which resorts to contact reduction using ammonia ss the reducing agent is considered to be highly promising because it is amply effective even when it is used for the treatment of exhaust gases of the type containing NO.sub.x and oxygen in relatively high concentrations. This method includes contacting a given exhaust gas containing NO.sub.x with ammonia at a temperature in the range of from 200.degree. C. to 400.degree. C. in the presence of a catalyst and permitting the NO.sub.x to react with ammonia thereby effecting conversion of NO.sub.x into N.sub.2. The reactions involved in this method are represented by the following formulas: EQU 6NO + 4NH.sub.3 .fwdarw. 5N.sub.2 + 6H.sub.2 O EQU 6no.sub.2 + 8nh.sub.3 .fwdarw. 7n.sub.2 + 12h.sub.2 o
for use in the treatments by the contact reduction method, there have been developed and suggested various kinds of synthetic catalysts including platinum-based catalysts, iron-based catalysts and copper-based catalysts. Of these catalysts, those other than platinum-based catalysts are generally deficient in activity at a low temperature below the level of 200.degree. C. Platinum-based catalysts, however, have a disadvantage that, with the progress of the reduction, they have their activity gradually degraded owing to the poisoning caused by sulfur oxides (hereinafter referred to as SO.sub.x) present in the exhaust gas under treatment and, moreover, the range of a temperature at which they exhibit high activity is narrow. Worse still, platinum-based catalysts are expensive.
The requirements which, from the industrial point of view, the catalyst being used in the contact reduction of NO.sub.x must fulfil to ensure advantageous performance in the contact reduction method are:
(1) the catalyst should be capable of retaining its high capacity for removing NO.sub.x even at a low temperature, and PA1 (2) the catalyst should be produceable at low cost.
Extensive research has been conducted in search of catalysts which satisfy these requirements.